Combustion misfires lead to an increase of toxic substances emitted during operation of the engine and can, in addition, lead to damage of a catalytic converter in the exhaust-gas system of the engine. A detection of combustion misfires in the entire rpm and load ranges is necessary to satisfy statutory requirements as to onboard monitoring of exhaust-gas relevant functions. In this context, it is known that, during operation with combustion misfires, characteristic changes occur in the rpm curve of the engine compared to normal operation without misfires. Normal operation without misfires and operation with misfires can be distinguished from a comparison of these rpm curves.
A method operating on this basis is already known from German patent publication 4,138,765.
In this known method, a crankshaft angular region which is characterized as a segment is assigned to each cylinder. The segments are realized, for example, by markings on a transducer wheel coupled to the crankshaft. The segment time in which the crankshaft passes through this angular region is dependent, inter alia, upon the energy converted in the combustion stroke. Misfires lead to an increase of the segment times detected in synchronism with the ignition. According to the known method, a criterion for the rough running of the engine is computed from the differences of the segment times. In addition, slow dynamic operations such as the increase of the engine rpm for a vehicle acceleration are mathematically compensated. A rough-running value which is computed in this way for each ignition, is likewise compared ignition-synchronously to a predetermined threshold value. Exceeding this threshold value is evaluated as a misfire. The threshold value is dependent, as may be required, from operating parameters such as load and engine speed (rpm).
The reliability with which misfires can be detected with this method drops naturally to a greater extent the less individual misfires operate on the rpm of the crankshaft. The reliability of the misfire detection therefore drops with increasing number of the cylinders of the engine and with increasing rpm as well as decreasing load.